1. Field of the Invention
This invention relates to medical X-ray apparatus and more particularly to an improved radiographic collimator.
In medical diagnostic studies with X-rays a beam of a desired size and cross-sectional configuration is delineated by a device known as a collimator. Typically a radiographic collimator has adjustable longitudinal and transverse sets of X-ray absorbent diaphragms which control the so-called field size, that is the transverse configuration and dimension of, an X-ray beam.
A major consideration in medical X-ray diagnosis is to minimize the radiation dosage administered to a patient during any given radiographic study. Accordingly it is important that when a radiographic exposure is to be made, the collimator diaphragms are positioned to provide exposure of a sheet of X-ray film but to avoid an oversized field size. That is radiation should not pass through the patient in regions around or at least adjacent to the field of interest because such radiation does not produce any diagnostic information.
The delineation of a beam of appropriate size is complicated by the fact that the distance between the source of X-rays and the imaging device is a variable. As an X-ray tube is moved toward a film of a given size, the apex angles of a given rectangular beam must be enlarged if the entire sheet of film is to be exposed. Conversely, the apex angles must be reduced when the X-ray tube is moved away from the film or a patient will receive needless extra dosage because the beam is larger in cross-sectional dimension in the plane of the film than is necessary to expose the film. 2. Description of the Prior Art
Proposals have been made for automatically varying diaphragm settings to control field size as source-to-image distance (SID) varies. One such proposal is disclosed in U.S. Pat. No. 3,502,872 to Edwin A. Norgren, entitled "Automatic Shutter Control for an X-Ray Spot Filmer".
Appropriate diaphragm setting is a function not only of the SID but also of the size film which is to be used. Accordingly, sensors are provided in a Bucky tray, as an example, to sense the size of a film cassette in the tray, provide electrical signals to a collimator control so that the beam settings are of appropriate size for the film in the tray.
Collimators are typically mounted on X-ray tubes which are suspended from the ceiling by a ceiling tube amount or alternately supported within an X-ray table. In either event, it is necessary to counterbalance the weight of the collimator. In addition, especially with a collimator within the table, it is not only desirable that the collimator be light weight but also important that it be compact.